The diaphragm (DIA) is the major inspiratory muscle in mammals, and its activation pattern is unique in relation to other skeletal muscles, e.g., a daily duty cycle of 45% versus <15% for hindlimb muscles. Therefore, the DIA may be particularly responsive to disuse, especially inactivity. The essential elements of DIA neuromotor control are motor units, each comprised of a phrenic motoneuron and the muscle fiber it innervates. In response to altered use, we expect that there are adaptations at each level of neuromotor control, i.e., muscle fibers, neuromuscular junction, and motoneuron. The guiding hypotheses of the proposed research are that: l) adaptations in DIA neuromotor control occurring in response to altered use are apparent at the level of muscle fibers, neuromuscular junction, and motoneuron; 2) adaptations in neuromotor control are dependent on motor unit type, such that fast units (type H fibers) are more adaptive than slow units (type I fibers); and 3) coherency between motoneuron and muscle fiber activity is an important determinant for the adaptations in neuromotor control. These hypotheses will be examined using four experimental models: 1) Denervation - where the muscle is inactive, motoneurons increase their activity, and communication between motoneuron and muscle fibers is disrupted; 2) TTX Blockade - where the muscle is inactive, motoneurons increase their activity, and communication between motoneuron and muscle fibers remains intact, but incongruous; 3) Spinal Isolation - where the muscle and motoneuron are both inactive, and communication between motoneuron and muscle -fibers remains intact and coherent; and 4) - Compensatory Loading - where the muscle and motoneuron both increase their activity, and communication between motoneuron and muscle fibers remains intact and coherent. In the proposed studies, we will use a variety of techniques to address four specific aims. These techniques will include: measurement of isometric and isotonic contractile properties; measurement of muscle fiber SDH and actomyosin ATPase activities and MHC isoform expression; ultrastructural analysis of myofibrillar and mitochondrial volume densities in muscle fibers and the incidence of fiber injury; morphometric analysis of the neuromuscular junction on type-identified fibers; assessment of synaptic efficacy and neuromuscular transmission failure; and morphometric analysis of phrenic motoneurons.